1. Field of the Invention
The present invention relates to a control method of received power for controlling received power of an optical signal in optical communication, an optical transmission apparatus, an optical receiving apparatus, and an optical communication system having such a control function.
2. Description of the Related Art
A system for performing optical communication usually comprises an optical transmission apparatus for transmitting an optical signal, and an optical receiving apparatus for receiving the optical signal transmitted from the optical transmission apparatus.
An example of the system for performing optical communication of this kind is disclosed in Japanese Patent Publication Laid-open No. 10-243475. This optical communication system is a system suitable for, for example, an optical cordless remote control, in which optical signals are transmitted and received bidirectionally between a main device and a sub device. Specifically, the main device emits a reference signal of a constant output, and transmits this signal to the sub device. The sub device receives the reference signal, detects signal intensity from this reference signal, performs an output adjustment in accordance with this signal intensity, and transmits an optical signal in accordance with the output adjustment to the main device.
When receiving the reference signal from the main device, the sub device functions as a receiving apparatus. For this reason, the sub device comprises an optical receiving unit for converting the reference signal, which is the received optical signal, into an electric signal, and an AGC (automatic gain control) circuit for adjusting power of the electric signal output from the optical receiving unit to a constant level. In this way, it is possible to obtain effects such as constantly optimized power consumption in the sub device (receiving apparatus).
FIG. 3 schematically shows such a conventional optical communication system. An optical signal output from one optical transmission apparatus 101 is transmitted to one optical receiving apparatus 103 through a transmission path 102. The power of the optical signal is attenuated while passing through the transmission path 102. The optical receiving apparatus 103 has an optical receiving unit 103a for converting the received optical signal into an electric signal, and an AGC circuit 103b for adjusting the power of the electric signal to a constant level after the conversion.
Furthermore, FIG. 4 is a block diagram schematically showing the optical communication system for switching and receiving the optical signals output from a plurality of optical transmission apparatuses. The optical communication system in this drawing comprises three optical transmission apparatuses 111a to 111c, and each of the optical transmission apparatuses 111a (to 111c) outputs optical signals a (to c) by time division. The optical signals a to c from the three optical transmission apparatuses 111a to 111c are multiplexed by an optical multiplexer 112, and transmitted to the one optical receiving apparatus 113 via an optical transmission path 114. The optical receiving apparatus 113 receives the optical signals transmitted in a multiplexed state, and performs the same receiving processing and AGC as in FIG. 3 by time division.
In the case of the optical communication system comprising the plurality of optical transmission apparatuses 111a to 111c, every optical transmission apparatus might transmit a signal at a different level, and losses in the transmission paths might be different. In such a case, as schematically shown in FIG. 5, the levels of the optical signals a to c received by the optical receiving apparatus 113 are different with respect to every optical transmission apparatus.
However, in the conventional optical communication system, as described above, while the AGC circuit is required on the side of the optical receiving apparatus such as the sub device in order to make the received power constant, this AGC circuit can not control the level of the optical signal received by the optical receiving apparatus not to exceed an absolute maximum rating of the apparatus. In addition, when the power of the optical signal received by the optical receiving apparatus is lower than a minimum light receiving power, it is not possible to recognize such an abnormality on the side of the transmission apparatus. That is, in the case of the conventional optical communication system, the AGC circuit is exclusive to the gain control, and it is impossible to control to bring into a power value desired at the receiving end from the transmitting end or monitor a transmission state.
In addition, in the optical communication system provided with a plurality of optical transmission apparatuses as described above, if switching time when the optical signals output from each of the optical transmission apparatuses are time-divided is at high speed, the AGC circuit built in the optical receiving apparatus is also required high-speed response in accordance with it. However, even in the AGC circuit having such a high-speed switching function, an insufficiency of function still remains, as described above.
The present invention has been achieved in view of such a situation concerning control of received power in conventional optical communication systems, and an object thereof is to ensure power of an optical signal to be controlled by an optical receiving apparatus without using an AGC circuit in the optical receiving apparatus.
To attain the foregoing object, a control method of received power in optical communication, an optical transmission apparatus, the optical receiving apparatus, and an optical communication system in accordance with the present invention comprise the following means:
Firstly, a control method of received power in optical communication in accordance with the present invention is a control method in optical communication in which an optical signal is transmitted from a transmitting end to a receiving end, wherein information on the received power of said optical signal received by said receiving end is fed back from said receiving end to said transmitting end; and said transmitting end adjusts output power of said optical signal so that the received power at said receiving end may be constant. In this way, it is possible to widely control the power (level) of the optical signal at the receiving end, without using the AGC (automatic gain control) for making the received power constant at the receiving end. As a result, the received power can be controlled not to exceed an absolute maximum rating of the receiving end. In addition, it is possible to know an abnormality in a transmission state also at the transmitting end with the information fed back, when the received power is lower than a minimum light receiving power necessary at the receiving end.
Secondly, an optical transmission apparatus for transmitting an optical signal to an optical receiving apparatus in accordance with the present invention comprises information receiving means for receiving information on received power of said optical signal fed back from said optical receiving apparatus; and power adjustment means for adjusting output power of said optical signal on the basis of the information received by said information receiving means so that the received power of said optical signal received by said optical receiving apparatus may be constant. In this way, since the optical transmission apparatus transmits the power received by the optical receiving apparatus in a state already made constant, it is no longer necessary for the optical receiving apparatus to be equipped with the AGC circuit.
Thirdly, an optical receiving apparatus for receiving an optical signal transmitted from an optical transmission apparatus in accordance with the present invention comprises feedback means for feeding back information on the received power of said optical signal to said optical transmission apparatus. Since the power of the optical signal transmitted to the optical receiving apparatus is already made constant by giving the feedback, it is no longer necessary for this optical receiving apparatus to be equipped with the AGC circuit.
Fourthly, in an optical communication system for transmitting an optical signal output from an optical transmission apparatus to an optical receiving apparatus in accordance with the present invention, said optical transmission apparatus comprises information receiving means for receiving information on received power of said optical signal fed back from said optical receiving apparatus, and power adjustment means for adjusting output power of said optical signal from said optical transmission apparatus on the basis of the information received by the information receiving means so that the received power of said optical signal received by said optical receiving apparatus may be constant; and said optical receiving apparatus comprises feedback means for feeding back said information to said optical transmission apparatus. In this way, since the optical transmission apparatus transmits the power received by the optical receiving apparatus in a state already made constant, it is no longer necessary for the optical receiving apparatus to be equipped with the AGC circuit. Therefore, it is possible to widely control the power (level) of the received optical signal, without using the AGC circuit in the optical receiving apparatus. As a result, the received signal can be controlled not to exceed the absolute maximum rating of the optical receiving apparatus, which can not be controlled by the AGC. In addition, when the received power is lower than the minimum light receiving power necessary for the optical receiving apparatus, it is possible to recognize an abnormality in a transmission state also in the optical transmission apparatus, with the information fed back.
Fifthly, it is also a preferred mode that the constitution of the optical communication system disclosed in the fourth paragraph mentioned above comprises said plurality of optical transmission apparatuses, and an optical multiplexer for multiplexing optical signals output from the plurality of optical transmission apparatuses and transmitting them to said receiving apparatus, and said optical receiving apparatus has feedback means for feeding back said information to each of said plurality of optical transmission apparatuses individually, as said feedback means. In this way, the optical receiving apparatus does not need to use the AGC circuit for switching the received signal into high speed to perform gain adjustment on the received power, for the optical signals sent on a high-speed time-division system from the plurality of optical transmission apparatuses, unlike conventional systems.